Sometime around the year 500 small quantities of ash and lava erupted from St. Helens' north flank. This period ended with the emplacement of dacite domes, including Sugar Bowl around the year 800. The Sugar Bowl dome is a nearly circular protrusion of hypersthene-hornblende dacite, about 800 m in diameter, on the north flank of Mount St. Helens. The base of the dome is at an altitude of nearly 1580 m, and its top is at 1865 m, roughly 1085 m below the summit of the pre-1980 volcano. A shelf or shallow depression on the upper northeast portion of the dome probably is a small explosion crater, but could be part of the irregular upper surface of the dome.
The USGS has a seismic station here. Its acronym is "SUG" you can view its data log here:
http://www.pnsn.org/WEBICORDER/BETTER/pnsn_staweb/index.html